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- <title>THE BOOST MPL LIBRARY: Handling Placeholders</title>
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- <div class="section" id="handling-placeholders">
- <h1><a class="toc-backref" href="./tutorial-metafunctions.html#id48" name="handling-placeholders">Handling Placeholders</a></h1>
- <p>Our implementation of <tt class="literal"><span class="pre">twice</span></tt> already works with metafunction
- classes. Ideally, we would
- like it to work with placeholder expressions too, much the same as
- <tt class="literal"><span class="pre">mpl::transform</span></tt> allows us to pass either form. For example, we
- would like to be able to write:</p>
- <pre class="literal-block">
- template <class X>
- struct two_pointers
- : twice<<strong>boost::add_pointer<_1></strong>, X>
- {};
- </pre>
- <!-- @ example.append('typedef two_pointers<int>::type intstar2;')
- compile('all', pop = 1, expect_error = True) -->
- <p>But when we look at the implementation of <tt class="literal"><span class="pre">boost::add_pointer</span></tt>,
- it becomes clear that the current definition of <tt class="literal"><span class="pre">twice</span></tt> can't
- work that way.</p>
- <pre class="literal-block">
- template <class T>
- struct add_pointer
- {
- typedef T* type;
- };
- </pre>
- <!-- @ compile() -->
- <p>To be invokable by <tt class="literal"><span class="pre">twice</span></tt>, <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt> would have
- to be a metafunction class, along the lines of <tt class="literal"><span class="pre">add_pointer_f</span></tt>.
- Instead, it's just a nullary metafunction returning the almost
- senseless type <tt class="literal"><span class="pre">_1*</span></tt>. Any attempt to use <tt class="literal"><span class="pre">two_pointers</span></tt> will
- fail when <tt class="literal"><span class="pre">apply1</span></tt> reaches for a nested <tt class="literal"><span class="pre">::apply</span></tt>
- metafunction in <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt> and finds that it
- doesn't exist.</p>
- <p>We've determined that we don't get the behavior we want
- automatically, so what next? Since <tt class="literal"><span class="pre">mpl::transform</span></tt> can do this
- sort of thing, there ought to be a way for us to do it too — and
- so there is.</p>
- <ul class="toc simple" id="outline">
- <li><a class="reference" href="./the-lambda-metafunction.html" id="id49" name="id49">The <tt class="literal"><span class="pre">lambda</span></tt> Metafunction</a></li>
- <li><a class="reference" href="./the-apply-metafunction.html" id="id50" name="id50">The <tt class="literal"><span class="pre">apply</span></tt> Metafunction</a></li>
- </ul>
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